1. Field of the Invention
The present invention relates to a method of forming a periodic structure and surface treatment, and more particularly to a method of periodically forming minute ripples on a surface of a material by irradiating a uniaxial laser beam thereon, and a surface treatment for changing surface characteristics of a material by irradiating a laser beam and thereby forming a periodic structure.
2. Description of the Related Art
Recently, development of micromachines, constituted of components that are a couple of orders of magnitude smaller than those of existing machines, has been aggressively promoted. While an inertia force such as the gravity is proportional to a cube of an object size, a surface force is proportional to a square of the object size. Therefore, when operating small parts such as those of a micromachine, an influence of the surface force acting between two objects becomes more apparent, rather than an influence of the gravity. Particularly, it is known that a pull-off force (or coagulating force), originating from a surface tension (or meniscus force) of water produced by condensation of moisture in the atmosphere at an interface between two objects, exerts a dominant influence to a friction force acting therebetween (Ref. non-patented document 1). It is also known that the pull-off force can be significantly reduced by minute ripples on the surface (Ref. non-patented document 2).
Also, it has been reported that minute ripples provide remarkable effect in retention of a lubricant and reduction of friction wear, which leads to an extended life span of the parts (Ref. non-patented document 3), and accordingly there has arisen a demand for development of a technique of forming a nanoscale microstructure on a material surface.
Likewise, it is known that irradiating a linearly polarized laser beam of a fluence near an ablation threshold to a polymer results in formation of a grate-shape minute periodic structure (Ref. non-patented documents 4, 5 and 6). It has also been reported that the same technique applies to a metal and a semiconductor as well, and that changing an irradiation angle can change a ripple spacing of the periodic structure (Ref. non-patented documents 7 and 8).
In all these cases a periodic structure of wavelength order is spontaneously formed, however it takes place only within a laser spot. Accordingly, those methods can only be applied to a limited region. In the event that a method of extensively forming such periodic structure on various materials is established, such method will serve to improve tribological characteristics of the material. Further, employing a femtosecond laser beam allows applying the method to small parts susceptible to a thermal effect, as well as to extremely thin parts.
In addition, for example the non-patented document 9 discloses a method of splitting a high-intensity femtosecond pulse of a titanium-sapphire laser into two parts, so that the interference of the biaxial laser beam forms a minute periodic structure, and a method of scanning a material attached to an X-Y stage in synchronization with a repetition frequency of a laser beam, to thereby form a periodic structure on an entirety of the material.
[Non-Patented Document 1]
Yasuhisa Ando, Toshiyuki Tanaka, Jiro Ino and Kazuo Kakuta: Relationships of Friction, Pull-off Forces and Nanometer-scale Surface Geometry, Series “C” of JSME (Japan Society of Mechanical Engineers) International Journal, No. 2, Vol. 44(2001), p. 453.
[Non-Patented Document 2]
K. N. G. Fuller and D. Taber,: The effect of surface roughness on the adhesion of elastic solids, Proc. Roy. Soc. Lond., A, 345, (1975) P. 327.
[Non-Patented Document 3]
M. Maillat, S. M. Pimenov, G. A. Shafeev and A. V. Simakin, Tribol Lett., 4, (1998), P. 237.
[Non-Patented Document 4]
P. E. Dyer and R. J. Farley: Periodic surface structures in the excimer laser ablative etching polymers., Appl. Phys. Lett., 57,8(1990) P. 765.
[Non-Patented Document 5]
H. Hiraoka and M. Sendova: Laser-induced sub-half-micrometer periodic structure on polymer surfaces, App. Phys. Lett., 64,5(1994) P. 563.
[Non-Patented Document 6]
M. Bolle and S. Lazare: Submicron periodic structures produced on polymer surfaces with polarized excimer laser ultraviolet radiation, Appl. Phys. Lett., 60,6(1992) P. 674.
[Non-Patented Document 7]
A. E. Siegman, P. M. Fauchet: Stimulated Wood's anomalies on laser-illuminated surfaces, IEEE J. Quantum Electron., QE-20,8(1986) P. 1384.
[Non-Patented Document 8]
Yukimasa Minami and Koichi Toyoda: Incident-angle dependency of laser-induced surface ripples on metals and semiconductors, Review of Laser Engineering, No. 12, Vol. 28 (2000), p. 824.
[Non-Patented Document 9]
Ken-ichi Kawamura, Masahiro Hirano and Hideo Hosono: Fabrication of micro-gratings on inorganic materials by two-beam holographic method using infrared femtosecond laser pulses, Review of Laser Engineering, No. 5, Vol. 30 (2002), p. 244.
However, the method of utilizing the interference of biaxial laser beams described in the non-patented document No. 9 has the following drawbacks. According to the method it is imperative to split the laser beam to form biaxial laser beams, with additional requirements such as setting an optical path difference to be strictly identical and strictly synchronizing a laser scanning speed with a ripple spacing of the periodic structure. Accordingly, control of optical axes is extremely complicated, and the apparatus inevitably becomes complicated and expensive. Besides, the method can only be applied to a flat surface because of utilizing the interference of two optical paths in different angles, and if a table supporting the material shakes, the ripple spacing of the periodic structure becomes uneven.
Likewise, the methods of forming a periodic structure described in the foregoing non-patented documents 1 through 9 are not appropriate for forming a periodic structure having an accurate ripple spacing over an extensive area through a simplified process, and therefore any practical application of those methods has not been established, since any effect thereof has not been proven yet.
Accordingly, it is an object of the present invention to provide a method of forming a periodic structure utilizing a uniaxial laser beam, instead of the foregoing biaxial laser beams, on a surface of various materials. It is another object of the present invention to provide a surface treatment technique of irradiating the laser beam on a surface of various materials, so as to change the surface characteristics thereof.